FIG. 1A schematically illustrates parts of an example wireless communication device 100, wherein a radio transceiver (RF RX/TX) 110 is shared (in time) by first and second radio access control units (RAT1, RAT2) 130, 140. A control unit (CU) 120 manages the time sharing in accordance with requests (with various priorities) from the first and second radio access control units.
In this type of implementation of a wireless communication device, various problems may arise in connection to situations where both radio access control units request access to the radio transceiver for windows of time that (at least partly) overlap. Typically, if such requests are associated with different priorities, the request having the highest priority is granted access to the radio transceiver. If the requests are associated with equal priorities, access to the radio transceiver may be granted in accordance with some suitable sharing algorithm (e.g. round-robin).
The first and second radio access control units may relate to the same or different radio access technologies, the same or different radio access systems, and/or the same or different subscriptions (e.g. implemented by subscription identity modules (SIMs)).
WO 2010/002337 A1 discloses priority handling between a measurement gap procedure and an uplink data transmission procedure in an evolved UTRAN when the the UE may use when performing measurements, i.e. no transmissions, neither uplink nor downlink transmissions, are scheduled during these periods.
A different situation is when the UE autonomously establishes a communication gap which the network will not be aware of. Such situations will be elaborated on more in the following.
In a typical example, a packet switched session (e.g. in accordance with the Universal Mobile Telecommunication Standard—Long Term Evolution (UMTS LTE)) may be ongoing for the first radio access control unit, when a higher priority request to use the radio transceiver is received from the second radio access control unit (e.g. controlling operation in accordance with the Global Standard for Mobile communication (GSM)). A gap in the use of the radio transceiver by the first radio access control unit is then created, which may lead to problems (e.g. decreased throughput, retransmissions, missed scheduling opportunities, etc.) in relation to the packet switched session of the first radio access control unit.
A few situations where gaps in the use of the radio transceiver by the first radio access control unit arise will be described in the following.
Paging
Wireless communication devices (user equipments—UEs) that are idle tune in to the corresponding network node (base station) at predetermined occasions, paging occasions, to check whether they are getting paged by the network. The reason for getting paged may, for instance, be that there is an incoming call for the UE to receive.
While it is in idle mode, the UE is handling the mobility autonomously using neighbor cell information provided by the network. If the current camping cell becomes weak and there is a stronger neighbor cell, the UE will change camping cell to the stronger neighbor. During this—so called—cell reselection, the UE is not monitoring paging and, hence, it may miss if it is getting paged at that moment. To prevent that the paging is missed due to interruption caused by cell reselection, radio access networks are usually repeating the paging one or more times until the UE responds.
All base stations in a so called location (or tracking) area for which the UE has registered are paging the UE. When the UE is reselecting to a cell in another location (or tracking) area, e.g. due to crossing some geographical boundary or changing to another radio access technology, it has to update the network regarding in which area it is via a Location (or Tracking) Area Update procedure. During the time when the UE is updating the location (or tracking) area, the radio access network will have outdated information regarding in which area the UE should be paged. To prevent that the paging is missed due to outdated location information, the radio access network usually repeats the paging in adjacent location (or tracking) areas if the UE does not respond to paging in the registered location (tracking) area.
Gaps in the use of the radio transceiver by the first radio access control unit may arise if the second radio access control unit needs to listen for pages in a paging occasion.
The paging occasions typically follow a so called paging cycle, which is configured by the radio access network node. The paging cycle length also depends on the radio access technology. Some example idle mode paging cycles include:
GSM—471, 706, 942, 1177, 1412, 1648, 1883, 2118 ms
WCDMA—640, 1280, 2560, 5120 ms
TD-SCDMA—640, 1280, 2560, 5120 ms
LTE—320, 640, 1280, 2560 ms
Circuit-Switched Fallback (CSFB)
Circuit switched fallback is an interim solution for supporting voice calls to UEs that are connected to UMTS LTE until VoLTE (voice over LTE, VoIP) and SRVCC (single radio voice call continuity) are supported in the networks.
This feature implicates that the UE can get paged in the UMTS LTE system for an incoming call in a legacy system (e.g. a GSM system), and can then get redirected to the legacy RAT (Radio Access Technology, e.g. GSM). This means that a UE can safely camp on, or be connected to, an UMTS LTE cell without missing any incoming calls.
Typically, the UE gets informed about whether CSFB is supported in the UMTS LTE cell when carrying out a combined registration for CS (circuit switched) and PS (packet switched) services. If CSFB is not supported, the registration will fail. The standard-compliant UE action when CS is not supported is to deactivate the support for UMTS LTE.
CSFB typically requires upgrades of legacy networks. Hence, in areas where UMTS LTE networks are rolled out, there might not always be CSFB support from the beginning How soon, and whether at all, CSFB will be supported depends on whether the operator is willing to invest in the legacy network.
If CSFB is not supported, gaps in the use of the radio transceiver by the first radio access control unit (e.g. UMTS LTE) may arise if the second radio access control unit (e.g. GSM) needs to listen for pages to allow UMTS LTE camping or connection while (at the same time) camping on a legacy RAT (e.g. GMS) to monitor CS paging.
Simultaneous GSM/LTE (SG-LTE)
SG-LTE is a solution that allows simultaneous GSM and UMTS LTE activities by having two separate radio transceivers and one or two baseband processing units. The UE can be engaged in UMTS LTE data traffic and (at the same time) support a voice call in GSM. Thus, a device supporting SG-LTE does rely on CSFB to allow UMTS LTE camping or connection. SG-LTE can be considered a special case of DSDA (dual SIM dual activity) where both SIMs are from the same operator (physically a single SIM).
Typically, the problems related to gaps in the use of the radio transceiver by the first radio access control unit do not arise in this case.
Simultaneous Voice and LTE (SVLTE)
SVLTE is similar to SG-LTE but more general in that any RAT providing CS and not only GSM can be used for offering CS service in parallel with UMTS LTE PS service.
Typically, the problems related to gaps in the use of the radio transceiver by the first radio access control unit do not arise in this case.
Single Radio-LTE (SR-LTE)
In SR-LTE a single radio transceiver is shared between UMTS LTE and a legacy RAT (e.g. GSM) in a time-division manner. The UE is connected to or camping on UMTS LTE while (at the same time) it is camping on a legacy RAT. When, for example, monitoring paging in the legacy RAT, reading system information, carrying out mobility measurements, doing a location area update, or receiving a call in relation to the legacy RAT, the radio transceiver is handed over to the legacy RAT and any UMTS LTE activities are getting punctured. A device supporting SR-LTE does not rely on CSFB to allow camping on or being connected to UMTS LTE. SR-LTE can be considered a special case of DSDS (dual SIM dual standby) where both SIMs are from the same operator (physically a single SIM).
Gaps in the use of the radio transceiver by the first radio access control unit (e.g. UMTS LTE) may arise if the second radio access control unit (legacy RAT, e.g. GSM) needs to perform any of the tasks exemplified above.
Monitoring Legacy RAT Using Available Additional Receiver
A UE capable of carrier aggregation may use an available receiver otherwise reserved for a secondary component carrier in carrier aggregation to monitor paging, carry out mobility measurements and/or read system information in the legacy RAT. As long as there is large enough separation between UMTS LTE uplink (UL) and legacy RAT downlink (DL) spectrum, the legacy RAT can be received concurrently with UMTS LTE transmissions on the UL. Hence, for this case the legacy RAT can be monitored without any impact on UMTS LTE performance.
Typically, the problems related to gaps in the use of the radio transceiver by the first radio access control unit do not arise in this case.
If the spectral separation between UMTS LTE UL and legacy RAT DL is not sufficient, collisions between UMTS LTE UL transmissions and legacy RAT reception needs to be avoided in order to prevent high energy leaking from the transmitter to the receiver and destroying the signal to be received, or even destroying the LNA (low-noise amplifier) used in the radio transceiver. In many cases, this will mean that UMTS
This situation may lead to that the problems related to gaps in the use of the radio transceiver by the first radio access control unit arise.
Depending on capabilities of the baseband and whether dual transmissions can be supported, it may also be possible to support functionality similar to SG-LTE or SVLTE with a single radio with two or more transceivers.
Dual SIM Dual Standby or Activity
In DSDS (dual SIM dual Standby) and DSDA (dual SIM dual activity) the UE is equipped with two SIM cards, and maintains connectivity (potentially) towards two different networks at the same time (typically for different operators).
For DSDA it is required that the UE uses separate radio transceivers for each connection, since, for example, it may use PS services simultaneously for both SIM identities, or PS service for one SIM and CS service for the other SIM. When one of the connections is terminated but the other still is active, the UE will be in idle mode for the SIM identity associated with the terminated connection. While in idle mode, it will monitor paging and carry out mobility management. For power saving reasons it may be attractive to use only one of the receivers in a time-division manner to maintain connectivity towards the first network and monitor paging in the second network (or for second identity in same network).
Thus, gaps in the use of the radio transceiver by the first (active connection) radio access control unit may arise when the second (idle mode) radio access control unit needs to monitor paging or carry out mobility management.
For DSDS it is not necessary to use two radio transceivers since it is assumed that the UE will be active only towards (at most) one network (or for one SIM identity) at any time, and will only monitor paging and carry out mobility management in the other network. With such a solution, DSDS is essentially similar to SR-LTE in that the radio transceiver is used in a time-division manner with puncturing of the ongoing connection when reading paging from the other network.
Thus, gaps in the use of the radio transceiver by the first radio access control unit (with active connection) may arise if the second radio access control unit (in idle mode) needs to perform any of the tasks exemplified above.
Impact of Puncturing on Link Adaptation
For single transceiver solutions, as well as for dual transceiver solutions in the case with too small spectrum separation, the UMTS LTE connection will be punctured, at least partially, during the time of reception in relation to the legacy RAT (or similar).
When puncturing the UMTS LTE connection, there will be an immediate throughput loss due to that scheduled transmissions to and/or from the UE cannot be carried out since the radio transceiver is tuned to another frequency, and also due to that HARQ (hybrid automatic request) acknowledgements (ACKs) for transport blocks received immediately before the gap cannot be transmitted and the base station may consequently retransmit the data although successfully received by the UE.
The puncturing may also have an impact on the residual BLER (block error rate) leading to retransmissions in higher layers (RLC—radio link control). Network vendors are typically using proprietary algorithms for link adaptation (i.e. radio condition dependent selection of transmission mode, modulation, and coding scheme). Typically, the link adaptation algorithms aim at maintaining a particular BLER for the transmitted channels. For instance, for PDSCH (physical downlink shared channel) the target may be 10% and for PDCCH (physical downlink control channel) it may be lower.
When no ACK/NACKs are received despite the UE has been scheduled, when CQI reports are missing, or when no transmissions are done by the UE despite having requested resources via scheduling request, the base station might assume, e.g., that the UE has not been able to decode the control channel (PDCCH), or that the base station has not been able to decode the transmission by the UE (PUCCH (physical uplink control channel) or PUSCH (physical uplink shared channel)). As a result the base station may increasingly use a more robust MCS (modulation and coding scheme) until the target BLER is reached.
For PDCCH this means that fewer control channels can be fitted within a control region of a fixed number of OFDM (orthogonal frequency division multiplex) symbols, or (alternatively or additionally) that the control region will have to be increased at the expense of the data region.
For PDSCH and also for PUSCH, this means that the throughput will be reduced. The throughput reduction is due to that, for a fixed allocation, the number of bits available for transmission may be reduced if falling back from a higher modulation to a lower, i.e. from 16QAM and 64QAM to QPSK and 16QAM respectively. Moreover, for a fixed number of available bits, the ratio of information bits to all available bits may decrease due to back-off, meaning that less information is transmitted and more bits are spent on encoding.
Thus, gaps in the use of the radio transceiver by the first radio access control unit, wherein the gaps are autonomously established by the second radio access control unit may be problematic.
Therefore there is a need for improved handling of gaps in the use of the radio transceiver by the first radio access control unit, wherein the gaps are autonomously established by the second radio access control unit. Preferably, any impact of the gap on performance related to the first radio access control unit is to be minimized, or at least decreased.